We are investigating the mechanism of the mitogenic response by selecting variant cell lines which do not respond to specific mitogens. Our rationale is that the missing function in mitogen-specific non-responsiveness may well be causal in the pathway of mitogenesis. We use the 3T3 cell as target. We have now successfully isolated EGF and TPA non-responsive 3T3 variants, and are currently characterizing the biochemical defects. One class of EGF non-responsive variants is missing EGF receptor. We are currently investigating a new variant which appears to have EGF receptors but has an attenuated mitogenic response. Our TPA variants now fall in two classes; some are also EGF non-responsive, while others respond to EGF. We plan now to examine the components of the "pleotypic response" in these variants. We also hope to select temperature-sensitive variants for proliferation.